The invention relates to a process for preparing mixtures of cyclic branched siloxanes of the D/T type, to the mixtures of cyclic branched siloxanes of the D/T type themselves, and to the processing of these siloxanes to give functionalized branched siloxanes and/or branched siloxanes.
Cited as a reference in relation to the M, D, T, Q nomenclature used in the context of this document to describe the structural units of organopolysiloxanes is W. Noll, Chemie and Technologie der Silicone [Chemistry and Technology of the Silicones], Verlag Chemie GmbH, Weinheim (1960), page 2 ff.
In the preparation of organomodified siloxanes, especially branched function-bearing siloxanes, a difficulty frequently encountered is that competing processes that take place simultaneously in the reaction matrix can adversely affect the quality of the desired product.
Condensation and equilibration are among these competing processes, which have to be considered separately according to the synthetic problem. A great challenge is the homogeneous distribution of branching sites along a siloxane chain, corresponding to avoidance of T-structured domains. As can be inferred from the literature, the breakup of homologous siloxane chains consisting of T units under acid catalysis in particular is difficult and hence in effect cannot be accomplished in the presence of sensitive functional groups. With regard to the reactivity characteristics of M, D and T units, reference is made to M. A. Brook, “Silicon in Organic, Organometallic and Polymer Chemistry”, John Wiley & Sons, Inc., New York (2000), p. 264 ff.
Especially in the preparation of branched siloxanes bearing reactive SiH groups, considerable efforts are therefore always made to reconcile the demand for uniform distribution of siloxane units as far as possible in a statistical manner with the demand for very substantial retention of the valuable silicon-bonded hydrogen.
Polyorganosiloxanes are prepared according to prior art by hydrolysis and condensation proceeding from methylchlorohydrosilanes having mixed substitution. Direct hydrolytic condensation of hydrogen-containing silanes, for example dimethylmonochlorosilane or methyldichlorosilane, is described, for example, in U.S. Pat. No. 2,758,124. In this case, the siloxane phase that separates in the hydrolysis is separated from the water phase having a hydrochloric acid content. Since this process is prone to gelation of the hydrosiloxanes, DE 11 25 180 describes an improved process utilizing an organic auxiliary phase, in which the hydrosiloxane formed is present as a separate phase dissolved in an organic solvent and, after separation from the acidic water phase and distillative removal of the solvent, is resistant to gelation. A further process improvement with regard to minimized solvent input is described by EP 0 967 236, the teaching of which involves first using only small amounts of water in the hydrolytic condensation of the organochlorosilanes, such that hydrogen chloride is driven out in gaseous form in the first step and can be sent directly to further end uses as a material of value.
Branched organomodified polysiloxanes can be described by a multitude of structures. In general, a distinction has to be made between a branch or crosslink which is introduced via the organic substituents and a branch or crosslink within the silicone chain. Organic crosslinkers for formation of siloxane skeletons bearing SiH groups are, for example, α,ω-unsaturated diolefins, divinyl compounds or diallyl compounds, as described, for example, in U.S. Pat. No. 6,730,749 or EP 0 381 318. This crosslinking by platinum-catalysed hydrosilylation downstream of the equilibration means an additional process step in which both intramolecular linkages and intermolecular linkages can take place. The product properties are additionally greatly affected by the different reactivities of the low molecular weight organic difunctional compounds that have a tendency to peroxide formation.
Multiple crosslinking of the silicone block of an organomodified polysiloxane with the organic block copolymer can be effected in various ways. EP 0 675 151 describes the preparation of a polyethersiloxane by hydrosilylation of a hydrosiloxane with a deficiency of hydroxy-functional allyl polyether, in which unconverted SiH functions are joined to the hydroxyl groups of the polyether substituents via an SiOC bond with addition of sodium methoxide. The increase in molar mass leads to broad scatter in the product properties, for example the viscosity. A similar approach to the formation of branched systems is described by U.S. Pat. No. 4,631,208, in which hydroxy-functional polyethersiloxanes are crosslinked by means of trialkoxysilanes. Both methods lead to intermolecular crosslinking of the polyethersiloxanes where it is not only difficult to control the increase in molar mass but where there are also associated unpredictable rises in viscosity. Following the aforementioned methods, what is obtained is not branching within the siloxane portion at constant molar mass, but crosslinking to give macromolecular multiblock copolymers.
Branching within the siloxane chain therefore already has to be effected in the course of production of the hydrosiloxane, in order to get round the described disadvantages of the crosslinking. Branches within the siloxane chain require the synthetic incorporation of trifunctional silanes, for example trichlorosilanes or trialkoxysilanes.
As known to the person skilled in the art, the rate of hydrolysis of the organochlorosilanes rises in the following series (C. Eaborn, Organosilicon Compounds, Butterworths Scientific Publications, London 1960, p. 179): SiCl4>RSiCl3»R2SiCl2>R3SiCl.
Therefore, in the hydrolysis and condensation reactions of trichlorosilanes, there is an elevated tendency to formation of highly crosslinked gels compared to the slower hydrolysis and condensation reactions of difunctional and monofunctional organochlorosilanes. The established processes for hydrolysis and condensation of dichloro- and monochlorosilanes are therefore not directly applicable to trichlorosilanes; instead, it is necessary to take indirect routes via multistage processes.
Building on this finding, it is also necessary to conduct the preparation of singly branched hydrosiloxanes by incorporation of not more than one trifunctional monomer per siloxane chain in a two-stage process according to the prior art. In a first step, a trifunctional low molecular weight hydrosiloxane is prepared by hydrolysis and condensation from 1,1,3,3-tetramethyldisiloxane and methyltriethoxysilane, as taught, for example, by DE 37 16 372. Only in a second step is equilibration then possible with cyclic siloxanes to give higher molar masses, as explained by DE 10 2005 004676. For further conversion—and therefore only in a third step—the singly branched hydrosiloxane thus prepared can be provided by the methods known per se for functionalization of siloxane compounds having SiH groups with organic substituents.
For synthesis of multiply branched hydrosiloxanes which, by definition, have more than one trifunctional monomer per siloxane chain, there are likewise two-stage syntheses in the prior art. In principle, it is possible to proceed from hydrosiloxanes and to subject the SiH functions, with addition of water and precious metal catalyst, to dehydrogenative conversion to silanols which are then condensed in turn with hydrosiloxanes. This procedure is described in U.S. Pat. No. 6,790,451 and in EP 1 717 260. Quite apart from the costs of the precious metal catalysis, the poor storage stability of the silanols, which have a tendency to autocondensation, makes it difficult to accomplish a reproducible, controlled process regime.
A further option described in U.S. Pat. No. 6,790,451 is that of preparing a copolymer from trichloromethylsilane or trialkoxymethylsilane with hexamethyldisiloxane or trimethylchlorosilane, also called MT polymer therein, which is equilibrated in a second step together with a polydimethyl(methylhydro)siloxane copolymer. The preparation of such MT polymers entails the use of strong bases or strong acids, in some cases in combination with high reaction temperatures, and results in prepolymers of such high viscosity that the neutralization thereof is made considerably more difficult and hence further processing to give end products of constant composition and quality is significantly limited.
According to EP 0 675 151, first of all, the hydrolysis and condensation of the SiH-free branched silicone polymer is conducted in xylene in such a way that the final occlusion of the precondensate is conducted with a large excess of hexamethyldisiloxane and, in the second step, the equilibration is undertaken with methylhydropolysiloxane to give a branched hydrosiloxane (preparation method 6, ibid.). As an alternative, the teaching of EP 0 675 151 relates to a procedure for preparation of non-SiH-functional branched siloxanes including merely a partial condensation of the methyltrichlorosilane used (preparation method 7, ibid.). However, these two strategies do not address the need for a universally utilizable preparation method for branched siloxanes.
WO2009065644 (A1) teaches a process for preparing branched SiH-functional siloxanes by reacting a mixture comprising a) one or more SiH-functional siloxanes, b) one or more SiH function-free siloxanes and c) one or more trialkoxysilanes with addition of water and in the presence of at least one Brønsted-acidic catalyst, wherein the reaction is conducted in one process step. The technical limits of this process become clear from the disclosure therein with regard to the conservation of the SiH functionality introduced into the system. This shows the need to work with at least two acidic catalysts (trifluoromethanesulfonic acid vs. trifluoromethanesulfonic acid and sulfuric acid ion exchange resin, ibid. examples 5 and 6) for sensitive SiH-functional branched siloxane structures, which makes the process extremely inconvenient and costly in terms of its industrial implementation.
There has already been speculation in the literature about the possible existence of siloxanes formed exclusively from D and T units. As stated by W. Noll in Chemie and Technologie der Silicone, Weinheim (1960), pages 181-182, D. W. Scott (J. Am. Chem. Soc. 68, 356, 1946) was the first to suggest that bicyclic compounds of siloxanes having D and T units derive from an extremely dilute co-hydrolysis of dimethyldichlorosilane and methyltrichlorosilane with subsequent thermal rearrangement. It was possible to isolate isomers in amounts of not even 1% from the viscous co-hydrolysate at bottom temperatures between 350 and 600° C., and they were then described by cryoscopic and elemental analysis with very high levels of uncertainty. Scott speculates that his compounds having D-T structures contain T structural elements joined directly to one another and not via D units. The interpretation of the results in Scott is based on the premise that all the SiC bonds present in the co-hydrolysate withstand the severe thermal treatment that he chose.
Makarova et al. (Polyhedron Vol. 2, No. 4, 257-260 (1983)) prepared 10 oligomeric methylsiloxanes having cyclic and linear segments by the controlled low-temperature condensation of siloxanes having SiOH groups and containing SiCl groups in the presence of organic amines such as triethylamine or aniline in benzene or diethyl ether as solvents, separated off the precipitated amine hydrochlorides, and washed and then fractionally distilled the crude reaction products. Subsequently, the bicyclic methylsiloxanes were subjected to pyrolysis at temperatures between 400 and 600° C., and the pyrolysis products were characterized by gas chromatography. The low molecular weight compounds used in the course of this study, for example hydroxynonamethylcyclopentasiloxane, hydroxyheptamethylcyclotetrasiloxane, dihydroxytetramethyldisiloxane, from the point of view of the silicone chemistry conducted on the industrial scale, are considered to be exotic species of purely academic interest.
More particularly, the pure-chain siloxane compounds of the D/T type defined in terms of molar mass that have been synthesized by this route are unsuitable for the production of organomodified siloxanes that are employed in demanding industrial applications, for example in PU foam stabilization or in the defoaming of plastics, etc. Active ingredients that effectively address such a field of use are always characterized by a broad oligomer distribution comprising high, moderate and low molar masses, since the oligomers present therein, depending on their molar mass and hence their diffusion characteristics, are very commonly imputed to have differentiated surfactant tasks in different time windows of the respective process. Specifically in the case of the branched organomodified siloxanes, due to the reaction characteristics of M, D and T units that have been discussed at the outset, however, a good oligomer distribution combined with a uniform distribution of siloxane units in a statistical manner as far as possible in the individual molecules can only be achieved when the starting material of the D/T type used already itself conforms to a distribution function. This is all the more true when the organomodification is effected via an intermediate bearing SiH groups.
Acknowledging this prior art, there is no apparent real solution for preparation of branched organomodified siloxanes.